“Coastal decisions by and large require long lead times, and it would be nice if we could wait for the science to clear up, but we can’t,” one scientist said.

As coastal communities prepare for the impacts of climate change, a new report warns that ice loss from Antarctica and Greenland could cause far more sea level rise than previously thought, and it says planners should not ignore that peril.

If planners are working with a mid-range projection of sea level rise, their efforts might protect coastal regions from the most likely scenarios depicted in climate models, but that still leaves a lot of risk, say the authors of the study published Monday in the journal Proceedings of the National Academy of Sciences.

“Coastal decisions by and large require long lead times, and it would be nice if we could wait for the science to clear up, but we can’t,” said Michael Oppenheimer, an atmospheric scientist at Princeton University and one of the authors of the study.

“If you knew there was a one-third or even 10 percent chance a plane would crash, you wouldn’t get on it. It’s the same with sea level rise,” he said.The authors write that, for planning purposes, it would be prudent to use scenarios that anticipate 6.5 feet of sea level rise by the end of the 21st Century—more than double the likely upper limit put forward by the Intergovernmental Panel on Climate Change.

That projection of 6.5 feet of sea level rise is based on a worst-case emissions scenario in which little is done to rein in greenhouse gases and the planet warms as much as 5 degrees Celsius (9°F) above pre-industrial times.

That amount of sea level rise could inundate nearly 700,000 square miles—almost equal to the entire land mass of Indonesia—”including critical regions of food production and displacement of up to 187 million people,” the authors write. “A [sea level rise] of this magnitude would clearly have profound consequences for humanity.”

Meeting the Paris climate agreement‘s ambitions—which would require governments to take steps that keep warming under 1.5°C—would result in as much as 2 feet of sea level rise. Another half degree, up to 2°C warming, would add 4 inches on top of that.